Project Summary/Abstract: Working memory is an essential cognitive faculty. Individual differences in working memory functioning can be quantified by working memory capacity (WMC). Higher WMC enables better performance in a diverse set of cognitive operations, including attention, reading comprehension, planning, and problem solving. There is evidence suggesting that higher WMC even confers the individual with the ability to better resist cognitive impairments in brain disorders. Despite its importance as a psychological construct, the neurophysiological underpinnings of WMC, however, remain not well understood. We will address this issue by pursuing Aim 1 in which we will investigate the individual differences in the task-related modulation of frontoparietal theta oscillations during working memory encoding and retention. Specifically, we will test the hypotheses that frontal theta power and frontoparietal theta coherence decrease with increasing working memory load during encoding and increase with increasing working memory load during retention and that theta modulation by working memory load during encoding and retention is positively correlated with working memory capacity. Research to date on the relation between neuronal oscillations and cognition tends to be correlative. Noninvasive neuromodulation provides a means to uncover the causal role of neuronal oscillations in cognition. In Aim 2 we will test the efficacy of tACS stimulation at theta frequency in enhancing task-related theta modulation and working memory capacity. Specifically, we will test the hypotheses that in-phase theta tACS stimulation of the frontoparietal network upregulates task-related modulation of theta oscillations in working memory and enhances working memory capacity and that individuals with low working memory capacity will benefit more from in-phase theta tACS stimulation than individuals with high working memory capacity.